Tracking Motion of a Writing Instrument

1. A method comprising receiving, from a writing instrument, light including a modulation frequency, sensing the light at each of two or more sensors each comprising sensitive pixels and generating a sequence of signals representative of the sensed light, determining the modulation frequency from the sensor signals using a phase lock loop, sampling a sensor signal at the times triggered by the phase lock loop, and determining a location of the writing instrument based on the sampled sensor signals.

2. The method of claim 1 also comprising applying a technique to increase a stability of the positions.

3. The method of claim 2 in which the technique is based on optics.

4. The method of claim 2 in which the technique is based on algorithmic processing of the generated signals.

5. The method of claim 4 in which the algorithmic processing comprises mapping the signal response of the sensors based on parameters associated with the writing instrument.

6. The method of claim 4 in which the technique is also based on optics.

7. The method of claim 2 in which the technique comprises an algorithmic technique that also reduces an effect of variations of intensity of the light based on other than dimensional effects.

8. The method of claim 2 in which the technique is implemented in analog circuitry.

9. The method of claim 2 in which the technique is implemented in digital hardware.

10. The method of claim 1 in which optics are configured to enhance the uniformity of signal response of the sensors.

11. The method of claim 10 in which the optics comprise a spherical lens.

12. The method of claim 10 in which the optics comprise an aspheric lens.

13. The method of claim 10 in which the sensors comprise analog sensors.

14. The method of claim 1 in which the sensors comprise pixel arrays, the signals are grouped in frames, and the method also comprises processing of multiple frames to cancel noise and increase a stability of the positions.

15. The method of claim 1 also comprising chopping the sensor signals at the modulation frequency.

16. The method of claim 15 also comprising applying opposite gains to each of the chopped signals depending on the on or off state of the light conveyed from the writing instrument that corresponds to the signals.

17. The method of claim 16 in which the chopped signals are integrated over time.

18. The method of claim 15 also comprising varying the modulation frequency.

19. The method of claim 1 in which characteristics of the conveyed light are used for synchronization between the light source and the sensors.

20. The method of claim 1 in which the times triggered by the phase lock loop are during the duration of the strong short pulse.

21. The method of claim 1 in which calculating the positions comprises determining an integer pixel location that is closest to a location along the linear array at which the maximum intensity of light has been received from the writing instrument, and finding a fractional center of gravity of a subarray that is centered on the integer pixel location.

22. The method of claim 1 in which determining the location comprises calculating positions of the light on each of the sensors at a resolution that is higher than a pixel resolution of the sensor.

23. A method comprising receiving, from a writing instrument, light including periods of lower frequency modulation and bursts of higher frequency modulation, sensing the light at each of two or more sensors each comprising sensitive pixels and generating a sequence of signals representative of the sensed light, using the sensor signal associated with the higher frequency bursts to lock onto a modulation clock, and determining a location of the writing instrument based on the sampled sensor signals.

24. Apparatus comprising a sensor comprising a linear array of sensitive pixels configured to receive light from a moving writing instrument and generate signals representative of the light, optics that are configured to enhance optical power of the light received from the writing instrument and that enable calculation of a position of the light along the linear array of the sensor at a resolution that is higher than a pixel resolution of the sensor along the linear array, and algorithmic processes that enhance immunity of the calculation to instability in the signals and variations in the intensity of the received light, in which the processes determine the integral power of the overall signal distribution on the sensor and calculate a position of the light at a resolution that is higher than the resolution of the pixels based on half of the integral power position.

25. Apparatus comprising a sensor comprising a linear array of sensitive pixels configured to receive light from a moving writing instrument and generate signals representative of the light, optics that are configured to enhance optical power of the light received from the writing instrument and that enable calculation of a position of the light only along the linear array of the sensor at a resolution that is higher than a pixel resolution of the sensor along the linear array, and algorithmic processes that enhance the immunity of the calculation to instability in the signals and variations in the intensity of the received light, in which the processes use a polynomial approximation on the signal distribution and calculate a position of the light at a resolution that is higher than the resolution of the pixels as a function of approximated maximum.

26. The apparatus of claims 24 or 25 in which the optics comprise a ball lens or an aspherical lens.

27. The apparatus of claims 24 or 25 in which the optics include a single spherical lens and the lens and the corresponding sensor are together configured to enhance the optical power of light received at large angles or longer distances or at disadvantageous positions of the writing instrument.

28. The apparatus of claims 24 or 25 in which the optics include a special lens configured to enhance optical power of the light received from a location on the X-Y surface that is beyond a predetermined position.

29. The apparatus of claims 24 or 25 in which the optics include two cylindrical lenses, one nearer the sensor to project light horizontally onto the sensor, and the other positioned to collect light in the Z-axis dimension, the other lens having a body that is bent around the first lens.

30. The apparatus of claim 25 also including a calibration procedure to produce parameters to be used in combination with data from the sensors.

31. The apparatus of claim 30 in which the calibration parameters correct for non linearity of the sensors, and the algorithmic processes use a quasi triangulation technique to determine a position of the writing instrument.

32. The apparatus of claim 31 in which the calibration parameters correct for non linearity of the sensors and the algorithmic processes determine the position of the writing instrument using polynomial series, and coefficients in these polynomials are determined during the calibration procedure.

33. A method comprising receiving light from a moving light source on a writing instrument as an indication of a location and path of the writing instrument on a two-dimensional writing surface, sensing the light at each of two or more sensors each comprising a linear array of sensitive pixels and generating a sequence of signals representative of the sensed light, and calculating from the signals positions of the light along the linear array of each of the two or more sensors at a resolution that is higher than a pixel resolution of the sensor by determining the integral power of the overall signal distribution on the sensor, and calculating a position of the light based on half of the integral power position.

34. A method comprising receiving light from a moving light source on a writing instrument as an indication of a location and path of the writing instrument on a two-dimensional writing surface, sensing the light at each of two or more sensors each comprising a linear array of sensitive pixels and generating a sequence of signals representative of the sensed light, calculating from the signals positions of the light along the linear array of each of the two or more sensors at a resolution that is higher than a pixel resolution of the sensor using a polynomial approximation on the signal distribution and calculating the positions as a function of approximated maximum.

35. The method of claim 33 or 34 in which the calculating also includes using algorithmic processes to enhance the immunity of the signals to variations in the intensity of the received light.

36. An apparatus comprising a sensor comprising a linear array of sensitive pixels configured to receive light from a writing instrument moving across an X-Y writing surface, the light including a modulation frequency, a phase lock loop configured to determine the modulation frequency from signals generated by the sensor, and optics that enable calculation of a position of the light along the linear array of the sensor at a resolution that is higher than a pixel resolution of the sensor along the linear array, in which the sensor signal is sampled at times triggered by the phase lock loop.

37. An apparatus comprising a sensor comprising a linear array of sensitive pixels configured to receive light from a writing instrument moving across an X-Y writing surface, the light including periods of lower frequency modulation and bursts of higher frequency modulation, and optics that enable calculation of a position of the light only along the linear array of the sensor at a resolution that is higher than a pixel resolution of the sensor along the linear array, in which the sensor signal associated with the higher frequency bursts is used to lock onto a modulation clock.

38. The apparatus of claim 36 in which the times triggered by the phase lock loop are during the duration of the strong short pulse.

39. The apparatus of claims 36 or 37 also including algorithmic processes that enhance the immunity of the signals to variations in the intensity of the received light caused by distance from or tilt of the writing instrument.

40. The apparatus of claims 36 or 37 in which the optics are configured to enhance optical power of the light received from the writing instrument.